Conventional heat exchangers include a heat exchanger core comprising a plurality of tubes supported between a pair of headsheets or header plates to which tanks are mounted. The tubes extend through holes in the headsheets and are either soldered or brazed to establish sealed fluid communication between the tanks. In most heat exchangers of this type, sheet metal fins are connected between the tubes to increase the heat transfer surface area. In use, a fluid is caused to flow through the tubes between the tanks so that heat transfer may occur between the fluid in the tubes and a second fluid, usually ambient air, flowing around the tubes and fins.
Since fin and tube heat exchangers of this type are pressurized and, when used for automotive applications, are subjected to relatively sever vibrations and torsional loads, it is important to maximize the strength of the soldered or brazed joint between the headsheet and the tubes to prevent failure of the heat exchanger core. With this objective in mind, it has become standard practice in the industry to form integral ferrules, or flanges, around the holes in the headsheet to extend the contact surface between the tubes and headsheet. Typically, the integral ferrules are formed substantially simultaneously as the holes are pierced in the sheet material out of which the headsheet is made.
When round tubes are employed little difficulty is encountered in forming integral ferrules out of corresponding or round holes in the headsheet. However, elongated tubes, generally referred to as flat tubes, are more commonly employed since they are thermodynamically more efficient. Since the holes in the headsheet must correspond to the cross section shape of the tubes, the holes are elongated and have parallel, spaced apart sides and rounded ends. Due to the small radius of the ends, it is difficult to form an integral ferrule around an elongated hole without cracks at the ends which are caused by overstretching of the sheet material. Cracks at the ends of the ferrule reduce the strength of the bond between the tube and headsheet and increase the likelihood of leaks.